爲什麼此C++程序不會爲第一個輸出顯示浮點數？

Question

我很好奇爲什麼如果代碼塊是一系列代碼塊中的第一個塊，C++爲什麼不將輸出顯示爲float。讓我來解釋......拿這個例子：

#include <iostream> 
#include<iomanip> 
using namespace std; 

int main() 
    { 
    double miles; 
    double fahrenheit; 
    double gallons; 
    double pounds; 
    double inches; 

    const double milesToKilometers=1.60; 
    const double fahrenheitToCelsius=5.0/9.0; 
    const double gallonsToLiters=3.9; 
    const double poundsToKilograms=0.45; 
    const double inchesToCentimeters=2.54; 



    // Get miles, then convert from miles to kilometers. 
    cout << "Please tell me how many miles you want converted to kilometers: 
    "; 
    cin >> miles; 

    if (miles < 0) 
     cout<<"You cannot enter negative numbers. Please run the program a 
     again and enter valid number." <<endl<<endl; 
    else 
     cout<<miles<<" miles is equal to "<<setprecision(2) 
     <<fixed<<miles*milesToKilometers<<" kilometers.\n\n"; 


    // Get fahrenheit, then convert from fahrenheit to celsius. 
    cout << "Please tell me how many degrees fahrenheit you want converted to 
    celsius: "; 
    cin >> fahrenheit; 

    if(fahrenheit < 0) 
    cout<<"You cannot enter negative numbers. Please run the program again 
    and enter valid number." <<endl<<endl; 
    else if(fahrenheit > 1000) 
    cout<<"You cannot enter numbers above 1000. Please run the program again 
    and enter valid number." <<endl<<endl; 
    else 
    cout<<fahrenheit<<" degree fahrenheit is equal to "<<setprecision(2) 
    <<fixed<< (fahrenheit-32)*fahrenheitToCelsius <<" celsius.\n\n"; 


    // Get gallons, then convert from gallons to liters. 
    cout << "Please tell me how many gallons you want converted to liters: "; 
    cin >> gallons; 

    if (gallons < 0) 
    cout<<"You cannot enter negative numbers. Please run the program again 
    and enter valid number." <<endl<<endl; 
    else 
    cout<<gallons<<" gallons is equal to "<<setprecision(2) 
    <<fixed<<gallons*gallonsToLiters<<" liters.\n\n"; 


// Get pounds, then convert from pounds to kilograms. 
cout << "Please tell me how many pounds you want converted to kilograms: "; 
cin >> pounds; 

if (pounds < 0) 
    cout<<"You cannot enter negative numbers. Please run the program again 
    and enter valid number." <<endl<<endl; 
else 
    cout<<pounds<<" pounds is equal to "<<setprecision(2) 
<<fixed<<pounds*poundsToKilograms<<" kilograms.\n\n"; 


// Get inches, then convert from inches to centimeters. 
cout << "Please tell me how many inches you want converted to centimeters: 
"; 
cin >> inches; 

if (inches < 0) 
    cout<<"You cannot enter negative numbers. Please run the program again 
    and enter valid number." <<endl<<endl; 
else 
    cout<<inches<<" inches is equal to "<<setprecision(2) 
    <<fixed<<inches*inchesToCentimeters<<" centimeters.\n\n"; 


return 0; 
} 

如果你運行它，並輸入任何英里數，（例如，10英里），這首碼輸出將顯示：

Please tell me how many miles you want converted to kilometers:10 
//input>>10 
10 miles is equal to 16.00 kilometers. 

的通知輸出說10英里不是10.00。現在觀察剩餘的輸出： 如果對於剩餘的碼塊輸入10，「華氏攝氏度，」「加侖到升」，「英寸到釐米」的輸出將顯示10.00，而不是10。另外，如果轉代碼塊的順序（例如，將'英里轉換爲公里'代替第一代碼塊的第二代碼塊），第一代碼塊將總是顯示一個整數（例如10）而不是浮點數（例如10.00）。

這是怎麼回事？

Answer 1

缺省值是代表精確的整數值的浮點值被不帶小數點的，沒有任何數字的（未顯示）小數點的右邊顯示。這就是爲什麼第一個是10。 插入setprecision(2)之後，浮點值是帶小數點和兩個數字在小數點的右邊顯示的所有。這就是爲什麼其餘的是10.00。如果您希望第一個顯示爲10.00，請移動setprecision(2)的插入，以便在顯示第一個值之前完成。

Answer 2

你在double聲明的變量;嘗試將它們聲明爲float這樣它將支持小數點，並且您不需要使用setprecision(n)。 但是！如果你的號碼中的一個回來跟小數點後的數字十億，並要限制隨後的小數，繼續在那裏折騰setprecision(n)回數的量。